A) In vitro replication system of yeast 2-Mum plasmid DNA mimics in vivo DNA replication. Crude extract made from cdc8 mutant cells shows temperature-sensitive DNA replication of 2-Mum plasmid in vitro. Using this system as a complementation assay, CDC8 gene product has been purified to homogeneity and found to be a single-stranded DNA binding protein. This protein interacts specifically to yeast DNA polymerase I which is a true DNA replicase. In the course of fractionation of in vitro DNA replication activity, a DNA primase activity has been detected, and particially purified. This DNA primase activity has been extensively characterized. B) A lot of attempts have been made to develop DNA transformation system of Drosophila melanogaster cultured cells. This is a crucial for isolation of DNA polymerase Alpha gene(s) from Drosophila melanogaster cells.